A semiconductor quantum well structure can be used to produce a laser having superior characteristics such as a low current threshold value where it is used as the active layer of the semiconductor laser and it can realize a waveguide having a low loss where it is used as a light waveguide. Furthermore, it has quite a wide range of possible applications to a variety of optical devices or modulators utilizing the electric field effect of its optical non-linearity or exciton absorption of a room temperature exciton.
In addition, the quantum well structure may be disordered by an impurity diffusion and thermal annealing thereby to become a layer of uniform composition. By utilizing this disordering, a planar waveguide and built-in type can be easily produced.
In a semiconductor laser or waveguide utilizing a quantum well structure of AlGaAs/GaAs series having a wavelength less than 1 micron, that is, the so-called short wavelength range, a number of examples utilizing disordering are reported. Because the lattice constant of Al.sub.x Ga.sub.1-x As does not vary with the Al composition ratio x, even if the quantum well structure becomes a layer of uniform composition by the disordering, lattice mismatching does not occur.
On the other hand, in a so-called long wavelength optical element in the 1 micron wavelength range, InGaAsP/InP series materials are usually used. In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y has a lattice constant varying with the composition ratio, that is, the values of x and y. As an example of a long wavelength range quantum well structure which is usually used, an In.sub.0.53 Ga.sub.0.47 As/InP quantum well structure is shown in FIG. 5 and its energy band diagram is shown in FIG. 6. In these figures, reference numeral 51 designates an In.sub.0.53 Ga.sub.0.47 As well layer, reference numeral 52 designates an InP barrier layer, reference numerals 53 and 54 designate InP cladding layers, reference numeral 61 designates a conduction band edge, and reference numeral 62 designates a valence band edge. The lattice constant of In.sub.0.53 Ga.sub.0.47 As layer 51 is approximately equal to the lattice constants of InP layers 52, 53, and 54.
When Si is diffused into the region 56 which is represented by the diagonal lines in the quantum well structure of FIG. 5, the portion where Si is diffused among the quantum well structure layers comprising In.sub.0.53 Ga.sub.0.47 As well layer 51 and InP barrier layer 52 is disordered to an In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y layer 57 of uniform composition, and it has a lattice constant different from that of the original In.sub.0.53 Ga.sub.0.47 As well layer 51 and the InP barrier layer 52. As a result, strain due to the lattice mismatching occurs at the boundary 55 between the portion where the quantum well structure is disordered and the portion where it is not disordered. This strain causes dislocations occurring at the boundary. For example, when the quantum well structure comprising above described materials is used for a the active layer of a laser and an active region is produced by disordering, the dislocations generated at the boundary increase during laser operation thereby to deteriorate the laser, resulting in poor reliability. In addition, because the strain due to the lattice mismatching suppresses the displacement of atoms and prevents disordering, a uniform composition layer is not obtained.
A prior art long wavelength, that is, 1 micron range semiconductor optical element has a quantum well structure comprising materials as described above. Therefore, when the disordering is carried out as described above, strain due to lattice mismatching occurs at the boundary between the disordered portion and the non-disordered portion, thereby effecting unfavorable influences on the element characteristics. This prevents realization of a device utilizing disordering of a quantum well structure in a one micron range, long wavelength optical element.